Studying the transition from non-invasive ductal carcinoma in situ (DCIS) to life-threatening invasive breast cancer (IBC) is listed in the top quartile of the 100 research priorities by the Institute of Medicine of the National Academies. This proposal explores innovative ways to better understand how early stage DCIS progresses to IBC. We recently found that when a DCIS lesion highly expresses two genes (ErbB2 and 14-3-3?), they readily progress to IBC, and we have identified key molecular alterations in tumor cells contributing to this progression to IBC with increased invasion. However, breast cancer progression is not a solo performance of tumor cells. It evolves as the result of sophisticated interaction between tumor cells and their surrounding microenvironment stromal components. Indeed, our preliminary data indicate that ErbB2 and 14-3-3? co-overexpressing mammary tumor cells dynamically interact with and activate stroma during tumor progression. Therefore, we hypothesize that ErbB2/14-3-3? co-overexpressing breast cancer cells alter their stromal components and convert normal stroma (tumor suppressive) to activated stroma (tumor- promoting), which reciprocally provide a permissive microenvironment to promote progression from DCIS to IBC. Here, we propose three Specific Aims to explore the roles of stroma activation in the progression of ErbB2/1433? co-overexpressing DCIS to IBC using mammary gland-specific 14-3- 3?.neu bitransgenic mouse models, human MECs, and patients' breast tumor samples. We will 1) Determine the effects of ErbB2 and 14-3-3? co-overexpression in mammary tumors on their stroma, and the contribution of activated stroma to tumor progression and metastasis; 2) Investigate the molecular mechanisms by which ErbB2 and 14-3-3? co-overexpressing mammary tumors induce stromal activation; 3) Examine the clinical relevance of stromal activation in ErbB2/1433?+++ DCIS from patients and its association with progression to IBC and metastatic breast cancer. Significantly, these proposed studies on the functional impact and mechanisms of the reciprocal interactions between ErbB2/14-3-3? co-overexpressing breast tumor cells and their dynamic stromal microenvironment will bring exciting new biological insights on how DCIS progressing to IBC and may impact on the clinical management of patients. This could open a new venue for development of future strategies to reverse the tumor-enhancing effects of the activated stroma by re-establishing a suppressive microenvironment to effectively prevent or reverse the deadly transition of DCIS to IBC.